1. Field of the Invention
This invention relates to a process for applying a wear and/or corrosion resistant coating to at least a portion of a metal and/or ceramic object. Thus the resistance may be resistance to mechanical abrasion but it may be resistance to corrosion, or any combination thereof.
The invention relates particularly, but not exclusively, to the application of a coating of hard alloys, metallic alloys, inter-metallic compounds, cermets, or ceramic materials or combinations thereof (hereinafter referred to as a special coating material) to a metal object.
2. Brief Statement of the Prior Art
Various proposals have been made and processes developed for applying a wear-resistant coating to tools, drill bits for example.
Specification no. GB 867,455 is concerned with a development of a spray-welding process in which a metal is sprayed onto a surface to be coated and the coating is then fused in place. The material to be sprayed is fed into a heating zone where it is melted or heat-softened, and from which it is then propelled in finely divided form, in a molten or heat-plastic condition onto the surface to be coated. The material being fed to the heating zone can be in the form of a powder or as a powder bonded together by a plastic material to form a wire. The fusing operation is performed in a furnace, or by means of heating torches applied directly to the coated surfaces. The development described was to employ a self-fluxing alloy in conjunction with a carbide in the form of a `fused aggregate`.
It was found that flame- or plasma-sprayed coatings were often of a porous nature and were not always bonded well to the substrate. Various attempts have been made to overcome these problems over a period of many years.
Specification no. EP 0,005,285A disclosed a process for applying a dense, hard and wear-resistant layer of cermets or ceramic material to a metal object by spraying-on of a matrix material and hard particles of cermets or ceramic material, followed by consolidation of the sprayed-on layer at high temperature and pressure, in which the sprayed-on layer is consolidated by isostatic compacting at a temperature of at least 1,000.degree. C., and a pressure of at least 1,000 bars, for at least half-an-hour. The consolidation step leads to a significant reduction in the porosity of the sprayed-on layer and, for many compositions of coatings, this and new compounds formed can lead to a more durable coating.
The process of EP 0,005,285A employs a binder metal of cobalt and/or nickel.